If the same message is simultaneously being edited by more than one user it's possible that changes you make are overwritten by others.
There is a notification system in place to help avoid this. When a user opens the message editor and the same message is being edited by another user a notification is shown to let them know of a potential conflict. If you see this notice you should coordinate with others before making changes to the email.
⚠️ If the same user login is shared by multiple users then this notification isn't shown. Not to mention that sharing the same user login and password is bad practice due to security reasons. New users logins can be added from the users page.
When you see a notification that another user is editing the message you just opened:
Exit the editor without making changes using the X button at the top right corner
Contact the other user and have them save and close the message.
Open the message once the other user confirmed that they are done with the changes and closed the editor.
There may be up to a one minute delay in the notification system so it's best to wait a minute before re-opening the message.